A hybrid electric vehicle (HEV) selectively utilizes multiple sources of energy in order to improve overall fuel economy, as well as to reduce certain vehicle emissions. An HEV includes a rechargeable energy storage system (ESS), which is ordinarily configured as a battery or a battery pack having a relatively high energy density. The ESS is electrically connected to at least one of the multiple energy sources. The multiple energy sources most often include a gasoline, diesel, or alternative fuel internal combustion engine and at least one electric motor/generator. Other HEV designs may alternately employ a fuel cell and/or another power source in place of or in conjunction with the internal combustion engine in order to further reduce vehicle emissions and improve operating range of the HEV.
In certain HEV designs, the drive wheels remain continuously connected to the driveline of the HEV to enable regenerative braking, thus providing a relatively efficient means of capturing useful braking energy. As is known in the art, regenerative braking can occur when an electric motor/generator is allowed to act as in its capacity as a generator during a regenerative braking event. When acting as a generator, the electric motor/generator can recharge the ESS while concurrently applying a negative torque to the drive wheels, thus slowing the HEV. Likewise, during normal driving operations the electric motor/generator can be operated as a motor, thus drawing power from the ESS and potentially powering the HEV and/or its various onboard systems.
As long as the drive wheels remain in continuous contact with the road surface, noise and driveline vibration are generally minimized within the vehicle interior. However, the coefficient of friction of certain road surfaces may be relatively low, such as on icy, snowy, oily, or wet pavement. The low coefficient of friction surface causes a disparity in the relative rotational speeds of the drive wheels with respect to the vehicle speed, or between the desired output torque and the actual torque imparted to the drive wheels.
In an HEV equipped with antilock braking system or ABS capability, slip occurring between the drive wheels and the road surface when a threshold braking force is applied to a brake pedal causes a controller to automatically activate the ABS. When the ABS is activated, a friction braking system is automatically pumped or cycled at a rapid rate in order to prevent the drive wheels from locking. However, depending on the particular road conditions as quantified by the coefficient of friction of the road surface, a perceptible driveline disturbance or vibration may result during the active ABS braking event, potentially leading to a less than optimal braking feel and performance.